Added Stack data structure.
Added Stack data structure.

celiafish/VisTrails,celiafish/VisTrails,minesense/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,VisTrails/VisTrails,minesense/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,Nikea/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,minesense/VisTrails,hjanime/VisTrails,hjanime/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,minesense/VisTrails,Nikea/VisTrails,Nikea/VisTrails,minesense/VisTrails,celiafish/VisTrails